NEW YORK — Humans are not the only animals that get drunk. Birds that gorge on fermented berries and sap are known to fall out of trees and crash into windows. Elk that overdo it with rotting apples get stuck in trees. Moose wasted on overripe crabapples get tangled in swing sets, hammocks and even Christmas lights.

Elephants, though, are the animal kingdom’s most well-known boozers. One scientific paper describes elephant trainers rewarding animals with beer and other alcoholic beverages, with one elephant in the 18th century said to have drunk 30 bottles of port a day. In 1974, a herd of 150 elephants in West Bengal, India, became intoxicated after breaking into a brewery, then went on a rampage that destroyed buildings and killed five people.

Despite these widespread reports, scientists have questioned whether animals — especially large ones such as elephants and elk — actually become inebriated. In 2006, researchers calculated that based on the amount of alcohol it takes to get a human drunk, a 6,600-pound (2,994kg) elephant on a bender would have to quickly consume up to 27 litres of 7 per cent ethanol, the key ingredient in alcohol. Such a quantity of booze is unlikely to be obtained in the wild. Intoxicated wild elephants, the researchers concluded, must be a myth. As the lead author said at the time, “People just want to believe in drunken elephants.”

If you are one who wanted to believe, a study published in April in Biology Letters might serve as your vindication. A team of scientists say that the earlier myth-busting researchers made a common mistake: They assumed that elephants would have to consume as much alcohol to get drunk as humans do. In fact, elephants are likely exceptional lightweights because they — and many other mammals — lack a key enzyme that quickly metabolises ethanol. The findings highlight the need to consider species on an individual basis.

“You can’t just assume that humans are just like every other mammal and the physiological abilities of all these mammals are comparable,” said Dr Mareike Janiak, a postdoctoral scholar in evolutionary anthropology at the University of Calgary and the lead author of the study. “Simply scaling up to body size doesn’t account for differences that exist between different mammal species.”

Humans, chimpanzees, bonobos and gorillas have an unusually high tolerance for alcohol because of a shared genetic mutation that allows them to metabolise ethanol 40 times faster than other primates. The mutation occurred around 10 million years ago, coinciding with an ancestral shift from arboreal to terrestrial living and, most likely, a diet richer in fallen, fermenting fruit on the forest floor.

To test whether other species independently evolved the same adaptation, Dr Janiak and her colleagues searched the genomes of 85 mammals that eat a variety of foods and located the ethanol-metabolising gene in 79 species. But they identified the same or similar mutation as humans in just six species — mostly those with a diet high in fruit and nectar, including flying foxes and aye-aye lemurs.

But most other mammals did not possess the mutation, and in some species, including elephants, dogs and cows, the ethanol-metabolising gene had lost all function.

“It was far more likely for animals that eat the leafy part of plants or for carnivores to lose the gene,” said Dr Amanda Melin, a molecular ecologist at the University of Calgary and a co-author of the study. “The takeaway is that diet is important in what we see happening in molecular evolution.”

Some results were unexpected. Tree shrews, for example, drink “copious amounts” of fermented nectar with ethanol content equivalent to weak beer, Dr Melin said, but they never show signs of inebriation. Yet tree shews do not share the same enzyme-producing mutation as humans. This implies that “there’s multiple, different ways to solve this problem,” she said.

Dr Nathaniel Dominy, a biological anthropologist at Dartmouth College who was not involved in the research, said the new paper “highlights the novel adaptations of humans by putting our metabolic proficiency in broader evolutionary context”. He said it also “exemplifies the power of comparative biology” for teasing out the underlying function of specific genetic traits.

The elephant findings, in particular, are “interesting but confusing,” said Dr Chris Thouless, the head of research at Save the Elephants, a non-profit in Kenya. Forest elephants today regularly seek and eat fruit, but their ancestors became grass eaters around 8 million years ago. Evidence indicates they then switched to a mixed diet around 1 million years ago.

“Maybe they lost the ability to efficiently metabolise alcohol, but either continued to have, or regained, a taste for and the ability to locate fruit,” Dr Thouless said. He compared it with people who have very low tolerance for alcohol but still desire and drink it.

While the new study reveals the means by which elephants and other mammals may become inebriated, it does not explicitly confirm the phenomena in nature.

“The persistent myth of drunken elephants remains an open and tantalising question, and a priority for future research,” Dr Dominy said. THE NEW YORK TIMES